Device for heating the paint in spray painting



Nov. 15, 1955 E; A. A. AXELSON 2,723,880

DEVICE FOR HEATING THE PAINT IN SPRAY PAINTING Filed July 18, 1951 h-naa V/W/V ESKIL AND ERS AUGUST XELMN IN VEN R United States Patent DEVICE FOR HEATING THE PAINT IN SPRAY PAINTING Eskil Anders August Axels on, Stockholm, Sweden Application July 18, 1951, Serial No. 237,470

Claims priority, application Sweden July 24, 1950 24 Claims. (Cl. 299-86) The present invention relates to spray painting installations, more particularly for use with paint which has to be heated before delivery to the spray gun.

It is previously known to heat the paint in spray painting before it is allowed topass through the paint spray gun. Generally, this is done either in a heat exchanger directly united with the gun, in which heat exchanger the hot compressed air used later for atomizing the paint has served as heat transmitting medium, or in an electric paint heater through which the paint has passed on its way to the gun.

Certain very conspicuous disadvantages are connected with both these arrangements. A heat exchanger united with the gun naturally increases the total weight of the gun, and therefore the capacity of the heat exchanger is limited to a maximum value practically realizable within the frame of the maximum dimensions of the. heat exchanger determined for reasons of suitability. However, in most cases, said value has proved too low. Furthermore, the hot heating medium must be conveyed directly to the gun, which, of course, involves great possibilities of heat losses, and moreover, makes heavy demands upon the heat-resistance and the insulation capacity of the supply conduits.

When electric paint heaters are employed there is always, particularly when cellulose paints are used, a great explosion risk in case some defect should arise in the device. Furthermore, in spite of the provision of thermostatic control of the current supply to the electric heating elements, it cannot be avoided that electric heating devices generally have no capacity of quickly changing their heat conditions. This is primarily due to the fact that the electric heat bodies and the resistance coils have a relatively large mass by which changes in the heat condition suffer from great inertia. As regards the heating of the paint, this means a great drawback inasmuch as at any pause in the spraying the temperature in the paint heater does not instantaneously fall, and thus the paint supply in the heater runs the risk of being heated to such an extent by the heat stored in the electric heating elements that it is destroyed and even burnt up within the heater.

If the paint heater is not placed close to the spray gun itself, or if it is not united with the same with the abovementioned drawbacks associated therewith, the temperature drop in the paint must be counteracted in some way or other at the passage of the paint through the supply conduits to the gun. This is generally obtained by using double paint hoses to the gun and by causing the paint to circulate through the hose and the paint heater by means of a circulation pump connected in the circuit. By this arrangement the paint temperature is held rather constant on the condition that the circulation pump does not change its pumping capacity due to wear or for some other reason which unfortunately is often the case. Such a device is therefore disadvantageous in the long run, all the more as the pump is very diificult to clean when another paint is to be used. Furthermore, recent developments have brought about a new aspect, it having been established that the passage of the paint through the pump might have a detrimental influence upon the paint itself. As a matter of fact, the pumping destroys the metal pigments in paints containing such pigments.

The present invention eliminates all the disadvantages pointed out above, and its chief object is to provide a device for heating paint for paint spraying.

The device according to the invention is substantially characterized by the fact that a heat exchanger separated from the spray gun and preferably working according to the counter-current principle, the heating medium of said heat exchanger consisting of a heated pressure medium, which preferably is also used for effecting the atomization of the paint, is arranged so as to heat the paint to a temperature suitable for the spraying which temperature is kept substantially constant throughout the conveying of the paint from the heat exchanger to the spray gun by means of suitable arrangements round the paint passage therebetween, said heat exchanger being controlled by a control device preferably united with the heat exchanger and working dependent on the pressure conditions in the path of flow of the pressure medium to the gun.

An embodiment of the device according to the invention is hereinafter described with regard to typical features and special advantages. The reference numerals which are found in the text, refer to the accompanying drawings.

In the drawings, Fig. 1 illustrates a sectional view of a heat exchanger united with the shifting or control device, while Fig. 2 is a longitudinal section of the new spray hose according to the invention, and Fig. 3 is an end view of the same hose.

According to the invention a heat exchanger preferably working according to the counter-current principle is used for heating the paint. In said heat exchanger there is provided in the space between two cylindrical and concentric tubes 1 and 2 a helically corrugated thin-walled tube 3 arranged concentrically with said tubes 1 and 2, the outer diameter of said tube 3 substantially corresponding with the inner diameter of the outer tube 2, its inner diameter being about the same as the outer diameter of the inner tube 1. In the inner helical passage 4 between the inner tube 1 and the corrugated tube 3 there is passed a hot medium, e. g. hot compressed air, which may be the same as that used for atomizing the paint. The paint is led through the outer helical passage 5 located on the other side of the corrugated tube 3 between said tube and the outer tube 2. The directions of flow of the hot medium and of the paint are preferably opposite to one another because in this case the heat exchanger has its greatest efficiency.

It has appeared in practice that the paint, in order to obtain a viscosity which is suitable for spray painting, should be heated to about 65 to 70 centigrades. A suitable and common value of the paint quantity in continuous paint feeding is about 500 g./min. As for economical and practical reasons the same medium, e. g. hot pressure air, should be used both as heating medium and as atomizing medium, and as under all circumstances the required quantity of such medium is about 400 to 500 liters/minute, said medium must have an initial temperature of to centigrades (dependent upon the initial temperature of the paint) ahead of the heat exchanger, in order that the paint quantity of said order of magnitude shall obtain the temperature suitable for spraying.

If the heat exchanger is not united with the spray gun, but is placed at a distance therefrom and possibly in the vicinity of the heating device for the hot medium, whether said medium is pressure air or vapor, stationary conduits may be used for supplying the heated gas to the heat exchanger, said conduits consisting of heat insulating tubes with great heat resistance, instead of rubber hoses or the like, which, due to their limited life, are very unsatisfactory in all respects at the temperatures in question.

The hot gas flows into the heat exchanger through the supply pipe 7 which is arranged in an insulated manner in the upper part 6 of the exchanger. This pipe enters the adjacent guide disc 8 on the inner tube 1 in the heat exchanger and terminates with its end abutting against a separating wall 9 arranged within the tube 1. The pipe 7 is provided Within the tube It With suitable outlet openings for the hot medium, through which said medium flows out into the tube 1 and flows upwardly and leaves through the holes 10 in the guide disc 8. The hot medium then passes through the helical passage 4- and through the communication holes 11 arranged at the bottom of the heat exchanger and passes to the outlet channel 12 which is connected to the T-tube 33 on the control device to be described hereafter. To this T-tube is connected the hose for conveying the atomizing medium to the spray gun. Due to the fact that the gas is first caused to how upwards within the tube 1 as viewed from the separating wall 9, and afterwards is immediately forced to pass downwards on the other side of the wall of the tube 1, an equilization or distribution of the temperature is obtained within the range in question which is especially advantageous during pauses in the spraying.

The paint enters the heat exchanger through the paint inlet 13 at the bottom and, as earlier mentioned, passes through the outer helical passage and leaves through the paint outlet 14 which via the paint hose, communicates with the spray gun.

The outer tube 2 as well as an insulating tube 15 preferably arranged outside said tube 2 are in each end rigidly connected, at the bottom with the lower ring piece 16 of the heat exchanger and, at the top, with the upper ring piece 17 thereof. The lower ring piece 16 is releasably secured on the outlet tube 12 for the heating medium by means of a nut 34, and the upper ring piece 17 is similarly secured on the upper part 6 of the heat exchanger by means of screws 18. The lower ring piece 16 is furthermore arranged to seal against a lower supporting ring for the helically corrugated tube so that the paint which is fed through the inlet 13, is prevented from forcing its way out between the ring piece 16 and the outlet tube 12. If the screws 18 and the nut 34 are untightened, the unit consisting of the parts 16 and 17 and of the tubes 2 and 15 connecting said parts may be withdrawn downwardly from the heat exchanger, whereby the paint channels on the corrugated tube 3 are laid bare and can be easily cleaned. This is of great importance, as thereby the heat exchanger can be easily cleaned before spraying with another paint and thus never need get a reduced capacity due to dried paint on the heat transmitting surfaces. Furthermore, air might enter the paint passage through the heat exchanger and thus the paint in the heating coil might dry and block the passage. Such a defect can easily be eliminated with the above mentioned device, which also is of great importance.

The spraying aggregate is provided with a special spray hose which will be described more in detail hereinafter. in this case the spray hose is arranged to maintain a substantially constant temperature in the paint in the hose during pauses in the spraying, and furthermore to provide that the paint quantity in the hose is heated to the correct temperature at the first spraying of the day. In order to satisfy these requirements already mentioned, the spray hose should of course also insure an allround heat insulation during the spraying proper when hot paint permanently passes through the same.

During pauses in the spraying the heat exchanger requires a small supply of heat in relation to the spray hose. On the other hand, the spray hose, which might be several meters long, needs a relatively great supply of heat in order to keep the temperature of the paint constant in the spray hose. Thus, if it can be arranged in such a manner that during spraying a relatively small 1621i quantity, or no heat at all, is supplied to the hose, dependent upon the length of the hose for compensating temperature losses in the paint, and a great amount of heat is at the same time supplied to the heat exchanger, and so that, during a pause in the spraying, the heat supply conditions are at the same time reversed to a certain extent, this will mean a great advantage.

This provision has also been made in the device according to the present invention, chiefly by the arrangement of a control or shifting device connected to the supply conduit for the hot medium, between the heating device for said medium and the heat exchanger. In Fig. l the device is shown united with the heat exchanger which is a combination offering many advantages. The control or shifting device, according to the principle indicated above, is intended to control the supply of hot medium to the heat exchanger as well as to the spray hose in response to actuation of the valve of the spray gun and to the pressure changes in the conduits, which changes are connected with the operation of the valve, so that at spraying and during pauses in the spraying the heating medium quantities which have been found suitable, are supplied to said heat exchanger and said spray hose. These quantities of hot medium may easily be adjusted to suitable values by throttling means or shunt couplings in the conduits.

The control device according to the embodiment shown operates in the following manner: An operating plunger which is displaceably arranged in the cylindrical inlet channel 19 for the hot medium coming from the heating device and which is arranged to be held in an upper position of rest by a spring 21, will move downwards at different pressures above and below the plunger in dependence upon said pressures and shift the heat supply conditions for the heat exchanger and the pressure hose to the operating condition. At the top, the operating plunger 20 communicates, via the supply conduit forthe heating medium connected to the inlet 19, with the heating device of said medium and, at the bottom, it communicates through conduit 25 with the same pressure medium leaving the heat exchanger, which pressure medium, due to pressure drop in the heat exchanger during flow, i. e. during spraying, has a lower pressure in conduit 25 than the medium at the upper part of the plunger 20.

When the trigger of the gun is pressed and the pressure medium for the atomization rushes out through the T- tube 33, a pressure drop instantaneously occurs in the cylinder space under the plunger 2t because the channel 25 opening in the bottom of the cylinder communicates with said T-tube 33 to which the air hose of the gun is coupled. The cylinder travels down and the pressure drop is too great to be compensated by the supply through the channel 22 which communicates with the inlet 19, and therefore the plunger 20 continues downwards against the action of the spring 21, and covers the lower opening of the channel 22 and the opening of the channel 23 which latter opening is situated on substantially the same level. Consequently, the deficit of pressure medium in the cylinder space under the plunger 20, which deficit is due to the outflow of the pressure medium when the spraying begins, causes said plunger to move downwards.

Immediately after the plunger 20 on its way downwards has covered the openings of the channels 22 and 23, it uncovers an opening 26 of a Wide channel which opening is situated above said first mentioned openings. Said wide channel communicates through a conduit 26' with the supply pipe 7 of the heat exchanger and delivers hot he ting medium to the heat exchanger. Since the heating medium, which in the contemplated case also is the spraying pressure medium, is supplied to the heat .5 exchanger and from there to the spray gun via the pressure medium hose, the operating plunger will be retained in its lower operative position in dependence upon the pressure difference between the pressure and heating medium above the same and that in the T-tube 33. This pressure drop naturally emanates from the flow of the medium through the heat exchanger and is determined by the flow velocity, the quantity of the medium and the dimensions of the channel in the heat exchanger.

When the spraying ceases and the valve of the spray gun is closed, the pressure drop in the outgoing conduit for the atomizing medium also ceases, and the spring 21 then presses the plunger 20 upwards whereby the channels 22 and 23 are uncovered again. The channel 22 now communicates with the cylinder space both above and below the operating plunger, whereby the channel 23 will be supplied with heating medium from said cylinder space. Preferably, the channel 22 has substantially the same dimensions as the channel 23, but in order that a pressure drop shall not arise in v the cylinder space below the plunger 24) at flow through'said channels, the channel 23 may be provided with an exchangeable throttling washer 24', whose flow area and thus also the heat supply to the hose 29 are suited to the length of the hose which is connected to the channel 23.

Naturally, one has to count with the fact that the plunger cannot seal completely against the cylinder walls because the heat conditions are such that a clearance is desirable between the plunger and the cylinder wall. For this reason, there will be a certain leakage, which may be of advantage from several viewpoints. For example, such a leakage may at the spraying deliver a certain quantity of hot medium to the channel 23 and thus to the hose 2) connected to said channel, said hose being included in the means for keeping the paint temperature constant. During pauses in the spraying the leakage may furthermore supply a certain quantity of hot medium to the heat exchanger so that the temperature of the latter does not fall below a certain value.

If said leakages areinsuflicientto attend to said functions, suitably dimensioned shunt conduits may be arranged to the channel 23, and -to,the supply passage 26' connected to the port 26 respectively, in order to supply extra heating medium, if desired.

In the device according to .theinvention illustrated in the drawing it is obvious that the shifting or ,control device is formed as a unit, which is secured to the upper part 6 of the heat exchanger by means of the screws 18 earlier mentioned, a heat insulating disc 27, e. g. of textile Bakelite, having been placed between said two-devices in order to prevent, during pauses in the spraying, thehigh temperature in the shifting unit from spreading to the paint via the upper part 6 of the heat exchanger. 2

The paint hose 28 which is connected to the paint outlet 14 of the heat exchanger and which may be of considerable length, must, as already mentioned, have substantially constant temperature (e. g. 65 centigrade), during pauses in the spraying. In order to make this pos-' sible in spite of the heating medium having a temperature of 150 centigrade, the paint hose 28 and a hose 29 for heating medium, which latter hose has substantially the same length as the paint hose and is connected to the channel 23, are placed in a common outer hose 30 which preferably is heat-insulated on the outside; The outer hose 30 is sealed in its'front end 31 by a seal 32 so that only the paint hose 28 passes through said seal 32 and is connected to the paint intake of the spray gun bymeans of a hose coupling. Thehose 29 is open at its front end and thus communicates with the space within the outer hose 30.

' During pauses in the spraying, an increased quantity of the hot medium passes through the heating hose 29 to the closed end 31 of the outer hose 30, and returns in the free space within the hose 30, whereby a heat equalization according to the counter-current principle isobtained between the hot currents flowing in opposite directions and the stationary paint in the paint hose 28' substantially maintains constant temperature. At'spraying, the outer hose30 together with any possible flow of heating medium from the channel 23 via the hose 29 serves as heat insulation and prevents a drop of the temperature of the paint.

Naturally, after the hot medium has been used to stabilize the temperature within the outer hose 30, said medium may be allowed to exhaust freely, from the outer end of the hose, but it may also be advantageous to blow off this exhaust medium around theoutside of the heat exchanger and thus assist in keeping the temperature therein constant. V "j A The device according to the invention has a great many advantages over earlier constructions, which make it superior to said constructions. The most conspicuous advantages may be summed up as follows:

1. No electric equipment in the paint heater and therefore reduced explosion risk.

2. The paint heater is simple to manufacture and easy to clean. I

3. During pauses in the spraying the temperature in the paint heater as well as in the paint hose remainssubstantially constant without use of circulation pump and double paint hoses, risks for defects disturbing the operation connected with the use of said devices being thus eliminated. I

4. Due to the very simple construction of the shifting device heat quantities of different volume may be quickly distributed to different parts of the device in dependence upon the valve of the spray gun and upon pressure drops arising in the system at the operation of said valve, without the risk of after-heating due to heat stored in the heating medium. I

5. Instead of increasing the paint viscosity to a value suitable for spraying by adding solvents, this increase can be obtained with the device according to the present invention by increased heat supply, whereby solvents are saved and the required number of sprayings is reduced because the paint then contains more film-forming matters per unit of space.

6. The drying period is also shortened due to the reduced quantity of solvents. The increased evaporation in the spray cone due to the hot paint also contributes to said reduction.

7 From the above description it might be obvious to any man skilled in the art, that a great many modifications of the typical device described by way of, example are possible within the frame ofthe invention. If it is not de sired to save heating medium by providing the control device but a simpler construction is desired, it is possible, by the arrangement of shunt conduits and leakage devices, to obtain a permanently constant heat supply to the heating hose and, to a certain degree, to the heat exchanger during pauses in the spraying. In certain cases the heating medium may be directly supplied to the heating hose 29 and, if desired, be mixed with cold medium to a suitable temperature so that thereby also the paint in the hose obtains a substantiallyconstant and desired-temperature. Furthermore, if it' is desired to obtain a control possibility, in order, for example, to give the atomizing medium a higher temperature, there is preferably arranged a controllable shunt between the intake 19 for the hot medium and the T-tube 33.

Having now described my invention, what I claim as new and desire to secure by Letters Patent is:

l. A heat exchanger for use in a spray painting installation including a source of pressure, a source oft-hot compressed gas, a gas feeding line, a source of paint connected to the source of pressure, a paint feeding line, a

. hot gas exhaust and a spray gun: said heat exchanger comprising a first channel system communicating at one end with the source of gas and at the other end with said spray gun, a second channel system communicating atone end with said spray source of paint and at the other end 2,72s,sso

tion includinga sourceof pressure, a source of hot compressed gas, a gas feeding, line, a source of paint connected to the source of pressure, a paint feeding line, a hot gas exhaust and a. spray gun: said heat exchanger ciomp'n'singa first channel system communicating at one end with the source of gas and at the other end with said spray gun, a second channel system communicating at one end with the source of paint and at the other end with the paint feeding line and therethrough with said spray gun, saidchannel systemsbeing in intimate heat exchange relationship, and control means for causing gas to flow substantially at all times direct from the source of gas to the gas exhaust and to flow upon actuation of the gun from the source of gas through the first channel system to the gas feeding line.

3. A heat exchanger as in claim 1, wherein the control means comprises a block having an inlet connected with the sourceof hot compressed gas, a first outlet connected withlthe first channel system and a second outlet connected with the gas exhaust, the cross-section of the first outlet being substantially greater than that of the second outlet.

4. A heat exchanger as in claim 1, wherein the control means comprises a block having an inlet connected with the source of hot compressed gas, a first outlet connected with the first channel system, a second outlet connected with the gas exhaust, andexchangeable throttling means in the second outlet for varying the cross-section thereof to be atmost equal to that of the first outlet.

5. A heat exchanger as in claim 1, wherein said control means comprises a cylinder, a piston slidable in the cylinder, a spring resiliently holding the piston at one end of the cylinder, means establishing a communication betweensaid one end of the cylinder and the source of hot compressed gas, a narrow restricted passage establishing a communication between the other end of the cylinder and the gun and means intermediate the ends of the cylinder establishing a communication between said one end and said first channel system when the piston is at said other end of the cylinder, said last-named means being covered by the piston when at said one end, the force of the spring and the size of the narrow restricted passage being so calibrated that the pressure of the hot compressed gas will overcome the combined forces of the spring and of the pressure at the gun when the latter is actuated for spraying thereby to displace the piston to said other end of the cylinder and cause hot compressed gas to flow through said first channel system.

6. A heat exchanger as in claim further comprising means by-passing said, piston for supplying a limited amount from the source of hot compressed gas to the gas exhaust when the piston is at said one end of the cylinder.

7. A heat exchanger for use in a spray painting installation including asource of pressure, a source of hot compressed gas, a first gas feeding line, a source of paint connected to the source of pressure, a paint feeding line, a second gas feeding line and a spray gun, the second gas feeding line and paint feeding line being in heat exchange relationship: said heat exchanger comprising a first channel system communicating at one end with the source of gas and at the other end with the first gas feeding line, a; second channel system communicating at one end with the source of paintand at, the other end with the paint feeding line and therethrough with the gun, said channel systems being arranged for counter-flow of the gas and paint therethrough and in intimate heat exchange relationship, and control means for the heat exchanger responsive to pressure comprising a cylinder, a piston slideable in the cylinder, a spring normally resiliently holding the piston at one end of the cylinder, means establishing a communication between said one end and the source of hot compressed gas, a narrow restricted passage establishing a communication between the other end of the cylinder and the first gas feeding line, means intermediate the ends of the cylinder establishing a communication between said one end of the cylinder and said first channel system when the piston is at said other end, said last named means being covered by the piston when at said one end, and means bypassing said piston for supplying gas from said source to said second gas feeding line when the piston is at said one end, the force of the spring and the size of the narrow restricted passage being so calibrated that the pressure of the hot compressed gas will overcome the combined forces of the spring and of the pressure at the gun when the latter is actuated for spraying thereby to displace the piston to said other end of the cylinder and cause hot compressed gas to fiow through said first channel system.

8. A heat exchanger as in claim 7 wherein the cylinder is provided in a block, said means bypassing the piston comprising a first passage in said block having one end communicating with the source of hot compressed gas and the other end opening into the cylinder in a position to be uncovered by the piston when at said one end of the cylinder and covered thereby when at said other end, and a second passage in said block having one end opening into said cylinder substantially at the same level as said other end of said first passage and the other end communicating with said second gas feeding line, the length of the piston relatively to the length of the cylinder and to the positions of the means establishing a communication between said one end and the first channel system and of the first and second passage openings into the cylinder being such that said means are covered by the piston prior to uncovering said openings when the piston moves toward said one end.

9. A heat exchanger as in claim 8 wherein the flow area of said second passage is smaller than that of said first passage.

10. A heat exchanger for use in a spray painting installation including a source of pressure, a source of hot compressed gas, a first gas feeding line, a source of paint connected to the source of pressure, a paint feeding line, a second gas feeding line, and a spray gun, the second gas feeding line and paint feeding line being in heat exchange relationship: said heat exchanger comprising a first channel system communicating at one end with the source of gas and at the other end with the first gas feeding line, a second channel system communicating at one end with the source of paint and at the other end with the paint feeding line and therethrough with the gun, said channel systems being in intimate heat exchange relationship, and control means for the heat exchanger responsive to pressure comprising a cylinder, a piston slideable in the cylinder, a spring normally resiliently holding the piston at one end of the cylinder, means establishing a communication between said one end and the source of hot compressed gas, a narrow restrictedpassage establishing communication between the other end of the cylinder and the first gas feeding line, means intermediate the ends of the cylinder establishing. a communication between said one end of the cylinder and said first channel system when the piston is at said other end, said last named means being covered by the piston when at said one end, and means bypassing saldpiston for supplying gas. from said source to said second gas feeding line when the piston is at said one end the force of thespringandthe size of the narrow restr cted passage being; so calibrated that the pressure of the hot compressed gas will overcome the combined forces of the spring and of the pressure at the gun when the latter is actuated for spraying thereby to displace the piston to said other end of the cylinder and cause hot compressed gas to flow through said first channel system.

11. A heat exchanger as in claim 10 wherein the cylinder is provided in a block, said means bypassing the piston comprising a first passage in said block having one end communicating with the source of hot compressed gas and the other end opening into the cylinder in a position to be uncovered by the piston when at said one end of the cylinder and covered thereby when at said other end, and a second passage in said block having one end opening into said cylinder substantially at the same level as said other end of said first passage and the other end communicating with said second gas feeding line, the length of the piston relatively to the length of the cylinder and to the positions of the means establishing a communication between said one end and first channel system and of the first and second passage openings into the cylinder being such that said means are covered by the piston prior to uncovering said openings when the piston moves toward said one end.

12. A heat exchanger as in claim 11 wherein the flow area of said second passage is smaller than that of said first passage.

13. A heat exchanger for use in a spray painting installation including a source of pressure, a source of hot compressed gas, a first gas feeding line, a source of paint connected to the source of pressure, a paint feeding line, a second gas feeding line, and a spray gun, the second gas feeding line and paint feeding line being in heat exchange relationship: said heat exchanger comprising a first channel system communicating at one end with the source of gas and at the other end with said spray gun, and a second channel system communicating at one end with the source of paint and at the other end with the paint feeding line and therethrough with said spray gun, said channel systems being arranged for counter-flow of the gas and paint therethrough and in intimate heat exchange relationship.

14. A heat exchanger as in claim 13 further comprising a flexible heat-insulating outer hose surrounding the paint and first gas feeding lines throughout a substantial length thereof between the heat exchanger and said spray gun, the first gas feeding line having an open end inside said outer hose remote from the heat exchanger, and the outer hose being closed at the end thereof remote from the heat exchanger and open adjacent the heat exchanger thereby to provide unidirectional flow of paint and gas and return flow of gas in heat-exchange relationship throughout the length of the outer hose.

15. A heat exchanger as in claim 14 wherein the open end of the outer hose communicates with the atmosphere. 16. A heat exchanger as in claim 14 further comprising a casing surrounding the heat exchanger, the open end of the outer hose communicating with the space inside the casing surrounding the heat exchanger.

17. A heat-exchanger as in claim 1 wherein said control means comprise a cylinder block, means insulatedly mounting said block on said heat exchanger, a cylinder in said block, a piston slidable in the cylinder, a spring resiliently holding the piston at one end of the cylinder, means establishing a communication between said one end of the cylinder and the source of hot compressed gas, means comprising a narrow, restricted passage establishing a communication between the other end of the cylinder and the gun and means intermediate the ends of the cylinder establishing a communication between said one end and said first channel system when the piston is at said other end of the cylinder, said lastnamed means being covered by the piston when at said one end, the force of the spring and the size of the narrow restricted passage being so calibrated that the 10 pressure of thehot compressed gas willovercome the combined forces of the spring and of the pressure at the gun when the latter is actuated for spraying thereby to displace the piston to said other end of the cylinder and cause hot compressed gas to flow through said first channel system.

18. A heat exchanger as in claim 17 further comprising means by-passing said piston for supplying a limited amount from the source of hot compressed gas to the gas feeding line when the piston is at said one end of the cylinder.

19. A heat exchanger for use in a spray painting installation including a source of pressure, a source of hot compressed gas, a gas feeding line, a source of paint connected to the source of pressure, a paint feeding line, a hot gas exhaust and a spray gun: a heat exchanger comprising an outer tube, a thin-walled helically corrugated tube inside the outer tube and concentric therewith to define a first channel therebetween, an inner tube inside said corrugated tube and concentric therewith to define a second channel therebetween, a supply tube for heat transfer medium opening into said second channel, and a partition wall positioned inside said inner tube intermediate the ends thereof adjacent the opening of said supply tube thereby to cause the hot medium entering said inner tube from said supply tube to change its direction of flow before entering said second channel and to resume its original direction upon entering said system.

20. A heat exchanger as in claim 19 wherein the heat exchanger comprises two telescopic units including a first inner unit comprising a first ring member, means integral with the first ring member for supporting one end of the corrugated tube, means supporting the supply tube inside the first ring member, the inner tube being supported on the supply tube inside the corrugated tube, and a gas outlet tube mounted on the corrugated tube at the end remote from the first ring member, and a second outer unit comprising a second and a third end ring members mounted at either end of said outer tube, the second one of said ring members cooperating sealingly with the first ring member when applied thereagainst and the third one of said ring members cooperating sealingly around said outlet tube, and a nut retaining said second and third ring members and said first ring member and outlet tube in sealing relationship.

21. A heat exchanger as in claim 20 having an insulating tube positioned outside said outer tube and mounted on said second and third ring members.

22. In a spray painting installation, a source of paint; a source of hot fluid; a spray gun; a flexible hose comprising: a first conduit connecting the source of paint to the gun, a second conduit connected to the source of fluid at one end and open at the other end and an outer sheath surrounding both said conduits and closed at the end thereof at which the second conduit is open and open to the atmosphere at the other end thereof; a source of atomizing gas and a gas feeding line connected thereto and to the gun, and control means responsive to pressure in the gas feeding line for controlling the flow of fluid through the second conduit.

23. The combination claimed in claim 22 wherein the fluid is a hot compressed gas.

24. The combination claimed in claim 22 wherein the fluid and the atomizing gas are derived from the same source.

References Cited in the file of this patent UNITED STATES PATENTS 1,845,690 Wait Feb. 16, 1932 1,861,091 King May 31, 1932 2,378,184 Carlson June 12, 1945 2,456,775 Fausek et al. Dec. 21, 1948 2,481,813 Bede Sept. 13, 1949 (Other references on following page) UNITED STATES PATENTS" Axelson Mar. 7, 19,50v Axelson. et a1. Mar. 7, 1-950 Arvinpz at al. Nov., 21, 1950 Grossman Mar. 27, 1.951 Wing May 1, 1951 Arvintz Aug. 28, 1951 12 Miller m. Mar. 25, 1952 Kopperschmidt Nov. 10, 1953 FOREIGN PATENTS Germany Jan. 2, 1903 France Feb. 9, 1907 France Jan. 27, 1943 

